According to the DEF UMAC, at what level do you need to refill the DEF tank?

Refilling the Diesel Exhaust Fluid (DEF) tank at a quarter tank is a recommended practice to ensure optimal operation of the vehicle's emissions system. Keeping the DEF level above this threshold helps prevent the vehicle from entering a derate condition, where engine power may be limited due to insufficient DEF supply for the selective catalytic reduction (SCR) system to function properly.

When the DEF tank is allowed to drop below the quarter tank level, there is a risk that the driver may run out of DEF, which is essential for meeting emissions standards. A quarter tank mark provides a sufficient buffer zone for drivers to plan refills conveniently before reaching a critically low level. This practice is an essential part of maintenance in vehicles utilizing SCR technology, ensuring compliance with environmental regulations and maintaining vehicle performance.